Theodolite Survey, Types, Parts and Functions, Measurement of the Horizontal angle

Theodolite Surveying

What is Theodolite?

Theodolite is the most important precise instrument used to measure the horizontal and vertical angles, to locate a point on the line and it as a wide application in surveying.

What is Theodolite Surveying?

The system of Surveying in which the angles are measured with the help of the theodolite instrument is known as theodolite surveying.

You can ask we have a compass to measure the horizontal angle, why should we use theodolite?

The answer for this is, in compass the measurement of the horizontal angle is with respect to the meridian that is the north, which is less accurate and also we cannot measure the Vertical angle using a compass.

Horizontal and vertical angle can be measured using theodolite accurately with the least count of 20 degrees.

Types of Theodolite

Theodolites are classified into two types,

Transit theodolite and non-transit theodolite.

In transit theodolite, the line of sight can be reversed by revolving the telescope through 180 degrees in the vertical plane but in non-transit theodolite, the telescope cannot be transited.

 Theodolite Diagram 

theodolite civiengg

Theodolite Parts and Functions

The transit theodolite consists of the following essential parts whose functions are to be known before starting the serving practice.

The telescope

 Vertical circle

 Index frame

 The standards

 The levelling head

 The two spindles

 The lower plate

 The Upper Plate

 Plate levels

 Tripod

 Compass

 Plumb Bob

 Striding level

The Telescope: It is an important part of the theodolite and is mounted on the spindle known as horizontal area.

Vertical circle: It is circular graduated arc attached to the trunnion axis of telescope consequently the graduated arc rotates with the telescope when the latter is turned about the horizontal axis.

The standards: The two standards resembling the letter A are mounted on the upper plates, the horizontal axis of the telescope is supported on these.

The levelling head:  the levelling head usually consists of two parallel triangular plates known as tribrach plates. The two tribrach plates are upper and lower tribrach. Upper tribrach has three arms carrying levelling screw and lower tribrach has a circular hole to suspend the plumb bob through it.

The two spindles:  The inner spindle is solid and conical fits into the outer spindle which is Hollow and ground conical in the interior.

The Lower plate: It is attached to the outer spindle. The lower plate carries a horizontal circle at its bevelled edge and is also known as a clamp screw.

The upper plate: The upper plate is attached to the inner axis and carries two extremities geometrically opposite upper plate support the standards.

The plate levels:  The upper plate carries two plate levels at a right angle to each other; one of the plate levels is kept parallel to the trunnion axis or horizontal axis.

Tripod: The theodolite is supported by the tripod. The tripod head carries at its upper surface an external screw to which the footplate of the levelling head can be screwed.

Permanent and Temporary Adjustments in Theodolite

  Permanent adjustment of transit is as follows:

 Adjustment of plate level.

 Adjustment of the line of sight.

 Adjustment of the horizontal axis.

 Adjustment of the altitude bubble and vertical index frame.

 Temporary adjustments of transit are as follows:

Temporary adjustments are those which are made it every instrument station when an instrument is shifted from one to another station. the temporary adjustments are:

Setting over the station.

Levelling up.

Elimination of parallax. 

 Setting over the station:

The operation of setting up includes centring of instruments over the station by a plumb bob and approximate with the help of tripod legs by moving tripod legs.

Levelling up:

 After having centered and approximate levelled the instrument, accurate levelling is done with the help of the foot screws and with reference to the plate level.

Elimination of parallax:

 Parallax is a condition arising when the image formed by the objective is not in the plane with the crosshairs. The parallax can be eliminated by two ways by focusing eyepiece for a distinct vision of crosshairs or by focusing objective to bring the image of the object in a plane of crosshairs.

 Important Terms and Definitions Used in Theodolite Surveying

The vertical axis:  the axis about which the instrument can be rotated in horizontal plane.

The horizontal axis:  the axis about which the telescope and vertical circle can be rotated in a vertical plane.

The line of collimation or line of sight:  it is the line passing through the intersection of the horizontal and vertical crosshairs.

 Centring: It is the process of placing the theodolite instrument exactly over the station point.

 Transiting: The process of turning the telescope in the vertical plane through 180 degrees about the horizontal axis.

Swinging the telescope: The telescope in the horizontal plane is known as the swinging of telescope. If the telescope is rotated in a clockwise direction it is known as right swing and in anticlockwise known as the left swing.

Face left: The face of the vertical circle is to the left of the observer, the observation of the angle is known as face left observation.

Face right: The face of the vertical circle is to the right of the observer, the observation of the angle is known as face right observation.

Measurement of the Horizontal angle by Repetition method 

Set the theodolite over the station Q’ make the temporary adjustment, and then the procedure discussed below is followed.

Repetation method

1.  With the help of upper clamp and tangent screw set 0 degrees reading on the vernier A note the reading on vernier B.

2.   Release the lower clamp and direct telescope towards point P clamp the lower clamp and bisect P accurately by the lower tangent screw.

3.  Release the upper clamp and turn the instrument clockwise about the inner axis towards R. Clamp the upper clamp and bisect R accurately with the upper tangent screw. Note down the readings of vernier A and B to get the angle PQR.

4.  Release the lower clamp and turn telescope clockwise to sight P, again clamping the lower one bisect P accurately by using a lower tangent screw. It should be noted that the vernier reading is not changed, in this operation since the upper plate is clamped.

5.  Repeat the steps to the desired number of trials.

6.   The average angle in one particular face will be equal to final reading divided by a number of trials.

7.  Repeat the steps 1 and 4 for another face and get the average angle for the face.

Measurement of the Horizontal angle by Reiteration method

 Set the theodolite over the station O’ makes the temporary adjustment, and then the procedure discussed below is followed.

Reiteration method

1.  With the help of upper clamp and tangent screw set zero reading on one vernier scale. Release the lower clamp and sight the point A and accurately bisect using the lower tangent screw.

2.  Release upper clamp and turn the telescope clockwise to sight point B. Bisect B   accurately using an upper tangent screw. Read both the vernier, the mean of vernier reading will give angle AOB.

3.  Similarly, bisect C, D. Thus closing the circle remain in a fixed position throughout the entire process each included angle is obtained by taking the difference between two consecutive readings.

4.  On final sight to A, the reading of the vernier should be is the same as the original setting if there is an error in reading it is due to slips etc,.

5.  If there is a small error it is distributed equally to all the angles for more accurate.

6.  If there is large repeat the procedure and take a fresher set of readings.

7.  Repeat the steps 1 and 4 for another face.

Measurement of Vertical angle using a Theodolite

 Set up two theodolites over the station P and make temporary adjustment as explained

1.  Release the vertical circle clamp and rotate the telescope in the vertical plane to set the object approximately and clamp it. Using the vertical circle tangent screw accurately basic the object read the angles in both vernier C and Dof thee vertical circle.

2.  The mean of the two gives the Vertical angle if the vertical circle is to the left of the observer then the readings are face left observation otherwise face right.

3.  In order to take the readings with the other face transit and swing the telescope to bisect object. Note down the angles in vernier C and D. The mean of the two gives Vertical angle.